The present invention relates generally to coated graphite articles such as crucibles and molds for use in high temperature metallurgical processes involving molten metal, and more particularly to such coated graphite articles wherein the coating inhibits carbon diffusion from the graphite article into the molten metal at melt temperatures up to about 2000.degree. C.
This invention was made with the support of the United States Government under contract No. DE-AC05-84OR21400 awarded by the U.S. Department of Energy. The United States Government has certain rights in this invention.
In metallurgical processes involving the melting of metals and alloys such as by vacuum induction melting or vacuum arc melting using consumable electrodes, the crucibles, as well as other metallurgical or foundry components, used in such processes are necessarily constructed of high-melting materials such as ceramics and graphite or water-cooled metal (copper). The melting of reactive metals such as uranium, zirconium, titanium, beryllium, and the alloys of such metals by induction melting techniques has been successfully achieved by using water-cooled copper crucibles but only in the production of relatively small melts of about 60 lbs (27 kg). On the other hand, using graphite as the construction material for the crucibles, molds and the like for the vacuum induction melting of such reactive metals provides for substantially increasing the size of the melts to about 500 to 800 or more lbs. However, this use of graphite causes undesirable carbide inclusions to be formed in the melt due to carbon from the graphite diffusing into the melt. Thus, while carbon contamination of the melted product is a major drawback to the use of graphite in high .temperature metallurgical processes involving molten metals and alloys, graphite is still particularly suitable for the construction of such metallurgical components or articles since it is readily machinable, possesses good electrical conductivity, provides excellent thermal shock resistance, and has high levels of refractoriness and strength at extremely high temperatures. Also, graphite components are less expensive than similarly sized copper or ceramic components.
Efforts to overcome this problem of carbon contamination of molten metals when graphite articles are used in the metallurgical processes has been previously addressed with some success being achieved by coating the surfaces of the graphite crucibles, molds, and other foundry articles contactable by molten reactive metals or alloys with various ceramics and refractory metals. These coatings have substantially reduced the carbon contamination of the melts at metal melting temperatures up to about 1500.degree. C. For example, graphite crucibles coated with either yttria or zirconia and used in the induction melting of uranium inhibited carbon contamination of the uranium by preventing carbon from diffusing through the coating at process temperatures up to about 1350.degree. to 1450.degree. C. However, at the higher process temperatures of 1400.degree. to 1450.degree. C., this coating failed to inhibit carbon diffusion and the molten uranium became contaminated by the carbon. Other coatings for graphite metallurgical articles included the use of two-layered coatings such as provided by a layer of niobium or tungsten under a layer of yttria and/or zirconia. These two-layered coatings successfully inhibited carbon contamination of the melt up to process temperatures of about 1500.degree. C., but at higher process temperatures carbon contamination of the melt was again present. Thus, while previously known graphite coatings have been successful for reducing carbon contamination of molten metals at temperatures up to about 1500.degree. C., these coatings have been found to be barely adequate for preventing carbon contamination of high melting reactive metals such as uranium (1300.degree. C.) and uranium alloys (1385.degree.-1500.degree. C.), beryllium (1280.degree. C.) and beryllium alloys and are totally inadequate for preventing carbon contamination of higher melting metals such as zirconium (1852.degree. C.) or titanium (1730.degree. C.) and alloys thereof.